In conventional ink jet recording heads, the vibrator which acts as a driving source for injecting an ink is composed of a thin piezoelectric film element. This thin piezoelectric film element normally comprises a thin piezoelectric film made of a polycrystalline substance and an upper electrode and a lower electrode arranged with the thin piezoelectric film interposed therebetween.
This thin piezoelectric film is normally made of a binary system having lead zircotitanate (hereinafter abbreviated as "PZT") as a main component or a tertiary system comprising the binary system having a third component incorporated therein. The thin piezoelectric film having such a composition may be formed, e.g., by sputtering method, sol-gel method, laser abrasion method, CVD method or the like.
A ferroelectric material comprising a binary PZT is disclosed in "Allied Physics Letters", 1991, Vol. 58, No. 11, pages 1161-1163.
Further, JP-A-6-40035 (The term "JP-A" as used herein means an "unexamined published Japanese patent application") and "Journal of The American Ceramic Society", 1973, Vol. 56, No. 2, pages 91-96 disclose a piezoelectric material comprising a binary PZT.
In the case where a thin piezoelectric film element is applied to an ink jet recording head, it is preferred that a thin piezoelectric film (PZT film) having a thickness of from about 0.5 .mu.m to 25 .mu.m be used. This thin piezoelectric film must have a high piezoelectric strain constant.
In general, it is reportedly necessary that the PZT film be subjected to heat treatment at a temperature of 700.degree. C. or higher to allow the crystal grains in the thin piezoelectric film to grow in order to obtain a thin piezoelectric film having a high piezoelectric strain constant. As the material constituting the lower electrode in the thin piezoelectric film element there may be used an electrically-conductive material such as platinum, titanium, gold and nickel.
JP-A-6-116095 describes crystal grains constituting a piezoelectric material. This patent discloses a process for the formation of a thin ferroelectric film which comprises applying a precursor solution of lead zircotitanate or lanthanum-containing lead zircotitanate to a platinum substrate which is oriented in (111) plane, characterized in that the application of the precursor solution is followed by heat treatment at a temperature of from 150.degree. C. to 550.degree. C., where a desired crystalline orientation is attained, further followed by calcining at a temperature of from 550.degree. C. to 800.degree. C. for crystallization, whereby a specific crystal plane of the thin film is preferentially oriented along the surface of the substrate according to the heat treatment temperature.
As a prior art technique concerning the present invention there is proposed a process for the preparation of a bulk piezoelectric ceramic as disclosed in JP-A-3-232755. As disclosed in this reference, it is said that a piezoelectric ceramic having a higher density exhibits better piezoelectric characteristics.
Further, JP-A-50-145899 discloses an example of the application of a bulk piezoelectric ceramic in the generation of a high voltage as in gas apparatus. This patent describes that a piezoelectric ceramic having pores having a diameter of from 4 to 10 .mu.m uniformly dispersed therein and having a specific gravity of from 90% to 93% based on the true specific gravity exhibits a percent discharge rate of 100%.
A conventional ink jet recording head comprising a thin piezoelectric film element is proposed in, e.g., U.S. Pat. No. 5,265,315.
In the case where a thin piezoelectric film (PZT film) having a thickness of not less than 1 .mu.m is formed, a problem arises that when the foregoing heat treatment is effected to obtain the foregoing high piezoelectric strain constant, cracking can occur in the film. As described in JP-A-3-232755, it is considered that a bulk ceramic having a higher density exhibits better piezoelectric characteristics. However, in order to make a good application of a film having a very high density to an actuator for ink jet recording head, etc., the optimum thickness of the piezoelectric film is from about 0.5 to 25 .mu.m. When a piezoelectric film having this thickness is produced at a single step, it is normally liable to cracking. If thin films are laminated to avoid cracking, it requires a prolonged production process which is industrially unsuitable.
Further, an approach for raising the thickness of the piezoelectric film by repeating a process which comprises applying a sol or gel composition to a substrate, and then calcining the material at a high temperature is disclosed in "Philips J. Res.", 47 (1993'), pages 263-285. However, this approach is disadvantageous in that the resulting thin piezoelectric film not only has a laminated interface that makes it impossible to provide good piezoelectric characteristics but also exhibits a deteriorated workability.
In general, a thin piezoelectric film is formed on a metal film which has been formed as a lower electrode on a substrate. However, a problem arises that the heat treatment effected during the formation of this thin piezoelectric film causes the substrate to be warped or distorted. Further, it is necessary that a good adhesion be established between the lower electrode and the thin piezoelectric film.
JP-A-50-145899 discloses a piezoelectric element comprising a bulk ceramic suitable for the generation of a high voltage. However, this differs in purpose from the present invention, which concerns a thin piezoelectric film element which can be applied to an ink jet recording head.
U.S. Pat. No. 5,265,315 discloses an ink jet recording head similarly to the present invention. However, this patent has no reference to the pores in PZT as piezoelectric film or the density thereof. Further, the proposed process for the preparation of the piezoelectric film comprises the use of sol-gel method and thus requires the lamination of a plurality of layers and a heat treatment process. Therefore, this proposal is industrially unsuitable.
In the above cited JP-A-6-116095, the orientation by X-ray diffraction wide angle method, i.e., orientation in the plane along the surface of the substrate is discussed. However, X-ray diffractometry of thin film is not discussed.
Further, if a piezoelectric element is used as an actuator for ink jet recording apparatus, etc., good piezoelectric characteristics are required. However, the relationship between crystal orientation and piezoelectric characteristics is not disclosed in JP-A-6-116095